Flat panel X-Ray detectors are in wide use in medicine. Most of these flat panel X-Ray detectors are based on a single light detector chip coupled with a scintillator. However, detectors of such a design are typically expensive. The single detector chip may be replaced by a plurality of less expensive optical sensors (e.g. CCD or CMOS) and lenses, which are arranged into a flat multi-camera array. X-Ray detectors including the multi-camera array may be less expensive in comparison with the single chip detectors since simpler sensors and lenses may be used. In multi-camera X-Ray detectors, each optical sensor acquires optical light irradiation from a segment of the scene as radiated from the scintillator. A complete image may be composed by stitching the plurality of partial images acquired by the plurality of single sensors.
The output image quality may be measured and assessed visually by visibility of the seam between the stitched partial images. Unfortunately, two neighbor images typically have intensity discrepancies in overlapping regions caused by differences in the relevant features of the sensors and their lenses, such as sensor's linearity of light-to-electrical transfer response, unity of optical performance of the lens, dark current, etc.
In addition, the acquired images may be distorted due to non-linearity of radiometric responses of the cameras. The camera radiometric response is the function providing correspondence of image irradiance values into measured intensity values that are output from the camera. Non-linearity of responses may cause image intensity not to be proportional to scene light intensity (irradiance). Off-the shelf CMOS sensors have substantially non-linear responses. Furthermore, response functions vary even between cameras of the same type.
It would, therefore, be desirable to produce an image having values that are nearly or even completely proportional to the scene irradiance. A linearization function to the radiometric response function of each of the cameras of the multi-camera flat panel detector may be estimated using known techniques. These known calibration techniques were developed for correcting non-linearity of optical devices of cameras, e.g. CMOS sensors and lenses. These known calibration techniques involve exposing the optical devices to light having known intensity levels and capturing a corresponding image for each intensity level, to obtain a plurality of calibration points, and calculating a linearization function based on the images and the corresponding known light intensity levels. Since the response of the scintillator, e.g. the relation between the irradiated visible light to the absorbed X-Ray energy level, is substantially linear, theoretically these techniques could be used for correcting of the non-linearity of the optical sensor and the lens of a flat panel X-Ray detectors. However, flat panel X-Ray detectors have X-Ray radiation as input. The optical sensors and lenses are separated from the X-Ray radiation by at least a scintillator layer, and the detector is typically enclosed in a casing. Calibration of the optical part alone will require opening of the casing of the detector for calibration. However, opening of the casing of the X-Ray detector is undesirable since it can, for example, lead to contamination of the system.
Linearity of radiometric responses of the cameras of the flat panel X-Ray detectors may assure the same sensitivity of the detector for different radiation doses. This linearity should be preserved during the entire life span of the X-Ray detector. This again, requires for a simple linearization process, which may be performed regularly in the field, without opening the cover of the detector.